Measuring apparatus



Sept. 22, 1942- I w. H. WANNAMAK ER ETAL MEASURING APPARATUS Filed Fab8, 1940 FIG.|.

INVENTOR 22 EARL A. KEELER WILLIAM H.WANNAMAKER Y Q NWW ATTORNEYPatented Sept. 22, 1942 t MEASURING APPARATUS William H. Wannamaker,Philadelphia County, and Earl A. Keeler, Norrlstown, Pa., assignors toThe Brown Instrument Company, Philadelphia, Pa., a corporation ofPennsylvania Application February a, 1940, Serial No. 317,894

13 Claims. (01. 175-183) The present invention' relates to the reductionor elimination of contact resistance in mercury switches.

object of the invention is to reduce the contact resistance to the flowof electric current present in a mercury switch so that such switchesmay be used in measuring circuits wherein the currents are very small.

Another object of the invention is to permit the use of an inexpensiveswitch in a measuring circuit requiring a switch of negligible contactresistance. In measuring circuits carrying currents of a low order ofmagnitude, it is necessary that switches connected in said circuits havea very low contact resistance in order to insure accuracy in themeasurements obtained. Switches designed for this use have in the pastbeen of the mechanical type employing wiping contacts and havenecessarily been expensive, because of the careful machine workrequired, or because of the special parts or materials needed to insureaccurate performance. Mercury switches of conventional construction havebeen considered unsuitable for such uses, because of their relativelyhigh and unstable contact resistance to small values of current. Incertain applications, it is desirable to use mercury switches containinga, small amount of oil, to reduce the adhesion between mercury and glassso that the mercury will move to make or break contact upon movement ofthe switch through a very small angle. Such mercury switches haveparticularly unstable contact resistance characteristics when used inlow current circuits.

In accordance with the present invention the difliculties encountered inthe past with mercury switches in such applications have been surmountedby applying a potential of suitable value across the switch contacts, sothat, upon closure,

the switch carries a current superposed upon the current being measured.The contact resistance of mercury switches to the flow of current once aflow of current has been initiated is known to be negligible, and it hasbeen found that the arrangement described effectively reduces andstabilizes the contact resistance of the switch. In accordance with ourinvention, any suitable current source, either direct or alternating,may

be superposed on the measuring current to obtain I ing'through parts ofthe circuit where its effect would be undesirable. Several means forachieving this etfect have been disclosed herein.

The potential applied across the switch in accordance with our inventionmust be suflicient so that the .voltage gradient which is estabilishedat the contact surfaces will be sufilcient to break down any film orother condition existing there and causing high resistance. Furthermore,the potential must not be so large that the current caused thereby willexceed the switch capacity. For example, a range of from 8 to 12 voltsis suitable for certain applications.

The various features of novelty which characterize our invention are setforth in the claims annexed to and forming a part of this specification.For a betterunderstanding of the invention, its advantages, and thespecific objects attained with its use, however, reference should bemade to the accompanying drawing and descriptive matter, wherein we haveillustrated and described several embodiments of our invena tion.

01 the drawing:

Fig. 1 is a diagrammatic representation of a multiple unit resistancethermometer system embodying our invention;

Fig. 2 is a wiring diagram of a bridge circuit for the resistancethermometer where in the superposed current is obtained from natingcurrent source and illustrates pref rre means of filtering thesuperposed alternating current from the circuit of the galvanometer usedtherein; and

Fig. 3 is a wiring diagram of another bridge circuit wherein thesuperposed current is obtained from a direct current source andillustrates a different means of preventing the superposed current fromadversely affecting the galvanometer operation.

In Figure 1 we have illustrated, more or less diagrammatically, the useof our invention in a measuring instrument employed to measure thetemperature at various locations in a building I, having resistanceunits 2, 3, 4, and 5 in different rooms thereof. Each of the saidresistance units has an appreciable temperature coefiicient ofresistance so that the resistance thereof varies in accordance with thevariations of the temperature to which they are subjected. The lowerterminal of each of these resistances, as seen in the drawing, isconnected to the terminal 8 of a bridge circuit designated generally bythe reference numeral 1. The upper. terminal of each resistance unit isconnected to two lead wires, one of which goes to the terminal 8 of thel perature units as noted hereinbeiore.

bridge], and the other to one terminal of a gal-' vanometer 9 which isadapted to indicate on'a suitable calibrated scale. (not shown) the t tof balance of the bridge I and thereby the temperature to which theresistance units 2, 3, l and 5 are subjected.

Each of the said upper lead wires from resistance unit 2 is connectedthrough an associated mercury switch mo: H, both of which switches aremounted on a bracket I2 which is pivotally supported on a rod i3.Thebracketis biased for rotation in a clockwise direction into;

engagement with a rod l8 by a spring ll coiled about a bolt l5 which isfastened to a panel 16 and passes through a suitable opening in thelower part of the bracket. The normal position -to which the bracket isso biased is such that vice 20 for the unit 2, that is, a device forselec-' tively connecting the unit 2 into and out of the circuit ofbridge I. When it is desired to measure the temperature in the roomcontaining the resistance unit 2, the pushbutton I9 is depressed,

causing the bracket 12 to move counter-clockwise and tilt the mercuryswitches ill and II so as to close them. The resistance unit 2 istherebyplaced in circuit with the bridge I and the temperature may bedetermined by noting the dethe'transiormer winding it means were notprovided to prevent such flow. The flow of such a large current in thiscircuit would cause an undesirable deflection or the galvanometerneedle, and might damage the instrument. In Figure 1,

. such iiow is prevented bythe switch 29, which is normally open, butmay be held closed when aof its corresponding selector switch 2|, 30,4|! or 50. If an additional current were not applied across the mercuryswitches, the deflection oi the galvanometer would be affected not onlyby the magnitude of the selected resistance, butby the this is anappreciable and a variable Iactor, the

flection oi the pointer 01 galvanometer 9 over its 1 associated scalewhich may be calibrated in tem- Selector switches 30, and similar to theselector switch 2|] just described are associated with each of theresistance units 3, I, and 5, respectively, and are provided to permitthe selective connection of each oi these resistance units into circuitwith the bridge '1.

After passing through the mercury switches, each of the upper leads fromthe various resistance units is connected through a common lead 2| or 22to its respective destination. Thepotential which is used in accordancewith the pres ent invention to reduce the contact resistance in themercury switches is applied across these leads 2! and 22. The potentialmay be obtained irom any convenient source as, for example, fromalternating current supply lines L and I? through a suitable transformer23. In order to isolate the direct current in the measuring circuit 1from the transformer 23, condensers 21 and 25 maybe connected in circuitbetween transformer 23 and the bridge conductors 2i and 22 as shown.

I When the selector button 19 is depressed, alternating current flowsthrough a circuit from one terminal oi the secondary winding oftransformer 23, through condenser 24, lead- 2i, switch i0, junction 26,switch H,-lead 22 and condenser 25 to'the other terminal'oi thetransformer secondary winding. When any other selector button isdepressed alternating current flows through the corresponding mercuryswitches in a similar circuit.

When all the selector switches are open, a relatively large currentwould flow from one terminal of the transformer secondary windingthrough branches 21 and 28 of the bridge, thence through thegalvanometer l and back to the other side of accuracy of the measurementwould be impaired. However, when an additional current is impressedacross the mercury switches as described, their contact resistance isreduced and stabilized, and the effect, if any, of the stabilizedresistance value upon the measurement may be eliminated by a propercalibration of the galvanometcr.

In Figure 2 we have illustrated a modification oi the arrangementdescribed in connection with Fig. 1 wherein the switch 29 may bedispensed with in preventing the flow of alternating current through thegalvanometer 8 and this end is obtained by the use of a pair ofelectrolytic condensers, 32 and 32, connected back to back across thegalvanometenterminals. The condensers 32 and 33 shunt the galvanometer,so that any alternating potential impressed across its terminals will beshunted through the condensers.

In Figure 3 we have illustrated a modification wherein the superposedcurrent is obtained from a a direct current source, and, wherein adiflerent means is used for preventing the ilow oi the added currentthrough the galvanometer. The solenoid 34 .o! a relay 35 is connectedbetween the opening the galvanometer circuit, thus preventing the flowof current through the galvanometer.

It will be apparent to those skilled in the art that other means may beused to pi event too great a current from flowing through thegalvanometer. For instance, a switching arrangement might be provided,such that one resistance unit would always be connected to the bridge,or an extra set' of contacts and a low resistance might be provided forthe oil, or normal, condition of the selector units 20, 30, 40 and 50. 1

While in accordance with the provisions of the statutes. we haveillustrated and described the best forms of embodiment oi our inventionnow known to us, it will be apparent to those skilled in the art thatchanges may be made-in the form of the apparatus disclosed withoutdeparting from the spirit of our invention as set forth in the appendedclaims and that certain features of our invention may sometimes be usedto advantage without a corresponding use of other features,

Having now described our invention, what we claim as newand desire tosecure by Letters Patent, is:

1. In combination, a measuring circuit including a mercury switch, meansfor impressing a current to be measured on said circuit, a source ofalternating potentiahmeans for impressing a potential from said sourceacross the terminals or said switch, and means for preventing the flowor direct current to said source.

2. The combination of claim 1, wherein said last-mentioned impressingmeans comprises conducting leads between the terminals of said sourceand the terminals of said switch, and said preventing means comprises acondenser in each of said leads.

3. In a circuit including a mercury switch and a direct currentmeasuring instrument, means for impressing a direct current to bemeasured on said circuit, means for impressing an alternating potentialacross said switch, and means for preventing the flow of alternatingcurrent through said instrument.

4. The combination of claim 3 wherein said last named means comprises anormally open, manually closable switch in series with said in strument.

5. A device as described in claim 3 wherein said last named meanscomprises a condenser in shunt with said instrument.

6. A device as described in claim 3 wherein said last named meanscomprises a relay having its winding in shunt with said switch, andhaving its contacts in series with said instrument, said contactresistance to the flow of electric currentcontacts being normallyclosed, but being opened upon energization of said relay when saidswitch is opened.

7. Measuring apparatus including in combination, an electrical bridgecircuit, an impedance variable in magnitude in accordance with themagnitude or a condition to be measured, means for connecting saidimpedance to said bridge cir-= cult, said connecting'means including amercury switch, and means for applying a-potential across the terminalsof said mercury switch to control the contact resistance thereof.

8. Measuring apparatus including in combination, an electrical bridgecircuit, a source or direct current for said bridge circuit, aresistance variable in magnitude in accordance with the magnitude of acondition to be measured, means for connecting said resistance to saidbridge circuit, said connecting means including a mercury switch, andmeans for applying an alternating potential across the terminals or saidswitch to nfeduce and stabilize the contact resistance there- 0 9.Measuring apparatus including in combination, an electrical network, anelectrical device variable in characteristic in accordance with themagnitude or a condition to be measured, means for connecting saiddevice to said network, said the terminals of said switch to control thecontact resistance thereof, and means for preventing the flow orfluctuating current through said instrument.

11. Measuring apparatus including in combination, an electrical network,an electrical device variable in characteristic in accordance with themagnitude of a condition to be measured, means for connecting saiddevice to said network, said connecting means including a mercuryswitch, and means for applying a potential across the terminals of saidmercury switch to control the contact resistance thereof. I

12. The method of substantially eliminating of a low order of magnitudein a switch connected in a circuit including said electriccurrent whichswitch when closed presents abnormal and unstable contact resistance tothe flow of such electric current which consists of impressing apotential directly across the terminals of said switch so that when saidswitch is closed the portion of said potential which is impressed onsaid circuit is due only to the potential drop across said switch, saidpotential being of suificient magnitude to overcome the conditioncausing the switch to present abnormal and unstable contact resistance,whereby upon closure of the switch a current is superposed on theelectric current normally carried by the switch.

13. In combination, a measuring circuit 'including a switch having thecharacteristic of presenting abnormal and unstable contact resistance tothe flow of electric current of a low order of magnitude, means forimpressing a current of a low order of magnitude to be measured on saidcircuit, and means for impressing a potential directly across theterminals of said switch so that when said switch is closed the portionof said potential which is impressed on said circuit is due only to thepotential drop across said switch, said potential being of suflicientmagnitude to overcome the condition causing the switch to presentabnormal and unstable contact resistance, so as to control the contactresistance of said switch.

WILLIAM H. WANNAMAKER. EARL A. KEELER.

